In recent years, in order to increase the information recording capacities of optical discs, blue semiconductor lasers (LDs) having light sources with shorter wavelengths are used. Objective lenses having high numerical apertures (NAs) also are employed. Furthermore, in systems employing those, techniques which apply two-layer discs are being put to practical use.
However, in an optical disc system using these, owing to a perturbation such as a tilt error of a disc from an optical axis of an optical pickup, a cover thickness error, or an interlayer aberration of a two-layer disc, the quality of a beam which reproduces signals is easily degraded, so that it becomes difficult to maintain good signal recording/reproduction characteristics.
As a related art which prevents such signal quality degradation, for example, it has been proposed to insert a compensation optical system in an optical path of a light beam irradiated onto an optical disc, in order to reduce coma aberration caused by a tilt error of the above mentioned disc (Japanese Patent Application Publication No. 2000-132854).
However, the above related art does not take enough account of various perturbations other than the tilt error of the disc, such as a cover thickness error and an interlayer aberration of a two-layer disc.
For this reason, to cope with the various perturbations mentioned above, the present applicant has proposed a method of improving the quality of a beam by controlling the phase distribution of a transmitting wavefront of the beam by the use of a liquid crystal device inserted in an optical path (Japanese Patent Application No. 2001-179245; hereinafter referred to as the prior application).
Namely, in the prior application, the phase distribution of the transmitting wavefront of the beam is controlled by means of two parameters, a thickness of a liquid crystal layer and an applied voltage. Specifically, the thickness distribution of the liquid crystal layer determines the shape of the phase distribution, while the value of the applied voltage determines an absolute amount of the phase.
However, in this prior application, there is a problem that since electrodes are formed along the shape of the liquid crystal layer, the thickness distribution of the liquid crystal layer actually does not accurately reflect the phase distribution given to the wavefront of transmitted light.
Namely, in a case where the electrodes are formed along the shape of the liquid crystal layer, in a small-thickness portion of the liquid crystal layer, since the distance between the electrodes which sandwich the liquid crystal layer therebetween is small, electric field strength becomes strong when a voltage is applied across the electrodes. Conversely, in a large-thickness portion of the liquid crystal layer, since a distance between the electrodes is large, the electric field strength becomes weak.
The phase amount that the liquid crystal layer gives to the transmitting wavefront depends on not only the thickness of the liquid crystal layer but also on this electric field strength.
Accordingly, in a case where a distribution occurs in the electric field strength in the liquid crystal layer, as parameters of the phase distribution of light being passing through the liquid crystal, it is necessary to take account of not only (1) the thickness of the liquid crystal layer and (2) the value of the applied voltage, both of which have been pointed out in the prior application, but also (3) electric field strength distribution.
However, since the electric field strength depends on the shape of the liquid crystal layer, as mentioned above, the shape of the liquid crystal layer also must be optimized to a further extent in order to control the phase amount of the transmitting wavefront to a desired value.
As mentioned above, in the prior application, in the construction in which the electrodes are formed along the shape of the liquid crystal layer, the shape of the liquid crystal layer does not accurately reflect the phase distribution given to the transmitted light, and if an accurate phase distribution is to be reflected, the electric field strength distribution also must be taken into account.
For this reason, in order to give a desired phase distribution to the transmitted light, the shape of the liquid crystal layer must be optimized to take the electric field strength distribution into account. Namely, there is a need for extremely complicated calculations from a calculation of the electric field strength distribution determined by the voltage value to be applied and the shape of the liquid crystal layer to a calculation of the phase distribution to be given to the transmitted light, and there is a problem that manufacturing of the device and device evaluations are difficult.
Thus, an object of the present invention is to provide a liquid crystal device which can give a desired phase distribution to transmitted light, without the need for complicated calculations, and which can also be easily manufactured and subjected to device evaluations, and to provide a manufacturing method for the liquid crystal device.